U.S. Pat. No. 6,869,474 to Perez-Pena et al, incorporated herein by reference, discusses extremely fast setting of cementitious compositions for producing cement-based products such as cement boards achieved by adding an alkanolamine to hydraulic cement such as portland cement, and forming a slurry with water under conditions that provide an initial slurry temperature of at least 90° F. (32° C.). Additional reactive materials may be included such as high alumina cement, calcium sulfate and a pozzolanic material such as fly ash. The extremely rapid set permits rapid production of cementitious products. Triethanolamine additions have been found to be a very powerful accelerator capable of producing formulations with relatively short final setting times with increased levels of fly ash and gypsum and without the need of calcium aluminate cements. However, formulations with triethanolamine also had relatively lower early-age compressive strength compared to cement board formulations containing the calcium aluminate cements.
Pending U.S. patent application Ser. No. 11/758,947 filed Jun. 6, 2007 of Perez-Pena et al, incorporated herein by reference, discusses extremely fast setting of cementitious compositions with early-age compressive strength for producing cement-based products such as cement boards achieved by adding an alkanolamine and a phosphate to a hydraulic cement such as portland cement, and forming a slurry with water under conditions that provide an initial slurry temperature of at least 90° F. (32° C.). Additional reactive materials may be included such as high alumina cement, calcium sulfate and a pozzolanic material such as fly ash. Again, all of the compositions contained a significant amount of hydraulic cement and gypsum.
U.S. Pat. No. 4,488,909 to Galer et al, incorporated herein by reference, discusses cementitious compositions capable of rapid setting. The compositions permit high speed production of carbon dioxide resistant cement boards by forming essentially all of the potential ettringite within about 20 minutes after the composition is mixed with water. The essential components of the cementitious composition are portland cement, high alumina cement, calcium sulfate and lime. Pozzolans such as fly ash, montmorillonite clay, diatomaceous earth and pumicite may be added up to about 25%. The cement composition includes about 14 to 21 wt % high alumina cement, which in combination with the other components makes possible the early formation of ettringite and other calcium aluminate hydrates responsible for early setting of the cementitious mixture. In their invention, Galer et al provided aluminates using high alumina cement (HAC) and sulfate ions using gypsum to form ettringite and achieve rapid setting of their cementitious mixture.
Ettringite is a calcium aluminum sulfate compound having the formula Ca6Al2(SO4)3.32H2O or alternatively 3CaO.Al2O3.3CaSO4.32H2O. Ettringite forms as long needle-like crystals and provides rapid early strength to cement boards, so that they can be handled soon after being poured into a mold or over a continuous casting and forming belt.
In general, Galer et al's rapid setting formulation suffers from several limitations. These limitations, as highlighted below, are even more of a concern for the production of cementitious products such as cement boards.
U.S. Pat. No. 5,536,310 to Brook et al disclose a cementitious composition containing 10-30 parts by weight (pbw) of a hydraulic cement such as portland cement, 50-80 pbw fly ash, and 0.5-8.0 pbw expressed as a free acid of a carboxylic acid such as citric acid or alkali metal salts thereof, e.g., tripotassium citrate or trisodium citrate, with other conventional additives, including retarder additives such as boric acid or borax, which are used to accelerate the reaction and setting time of the composition to overcome the disclosed disadvantageous of using a high fly ash content in cement compositions.
U.S. Pat. No. 5,536,458 to Brook et al disclose a cementitious composition containing a hydraulic cement such as portland cement, 70-80 parts by weight fly ash, and 0.5-8.0 pbw of a free carboxylic acid such as citric acid or an alkali metal salts thereof e.g. potassium citrate or sodium citrate, with other conventional additives including retarder additives such as boric acid or borax, which are used to accelerate the reaction and setting time of the composition to overcome the known disadvantageous of using a high fly ash content in cement compositions.
U.S. Pat. No. 4,494,990 to Harris discloses a cementitious mixture of portland cement e.g. 25-60 pbw, fly ash e.g. 3-50 pbw and less than 1 pbw of sodium citrate.
U.S. Pat. No. 6,827,776 to Boggs et al disclose a hydraulic cement composition comprising portland cement, fly ash, which has a setting time controlled by pH of an activator slurry of an acid, preferably citric acid, and a base which can be an alkali or alkaline earth metal hydroxide or salt of the acid component.
U.S. Pat. No. 5,490,889 to Kirkpatrick et al disclose a blended hydraulic cement consisting of water, fly ash (50.33-83.63 pbw), portland cement, ground silica, boric acid, borax, citric acid (0.04-2.85 pbw) and an alkali metal activator, e.g. lithium hydroxide (LiOH) or potassium hydroxide.
U.S. Pat. No. 5,997,632 to Styron discloses a hydraulic cement composition containing 88-98 wt. % fly ash, 1-10 wt. % portland cement and from about 0.1-4.0 wt. % citric acid. Lime to achieve a desirable minimum lime content of 21% is provided by the subbituminuous fly ash or the sub-bituminous fly ash in combination with a beneficiating agent. In addition to citric acid Styron uses an alkali source such as potassium or sodium hydroxide.
The final setting times of the cementitious mixtures of prior art products are typically greater than 9 minutes and can extend to 2-3 hours for standard concrete products. The final setting time is normally defined as the time in which the cementitious mixtures set to the extent that the concrete products made thereof can be handled and stacked, although chemical reactions may continue for extended periods.
The amount of high alumina cement (also known as calcium aluminate cement) in the reactive powder blend in prior art concrete products is also very high. Typically, the high alumina cement is greater than 14 wt % of the reactive powder blend.